The present invention is devoted especially to utilization in connection with the exhaust of the internal-combustion engine of small, lightweight aircraft, e.g., in the order of 100 to 150 H.P. In aircraft of this type, the occupant does not have the comfort of an enclosed cockpit or insulated cabin, but on the contrary, rides fairly close to the engine and is of course exposed to engine noise, which is quite high in decibels. Although this problem has been long recognized, a satisfactory muffling system has, so far as is known, not been devised, especially systems that can be purchased on the open market as distinguished from hand-crafted mufflers adapted for special circumstances. A further need for a satisfactory muffler in small aircraft environment is demonstrated by the problems related to excessive noise that can affect aircraft and occupant safety. For example, when excessive noise is present, it is difficult for the pilot to detect certain engine malfunctions, such as approaching stall. Moreover, excessive noise is known to create hearing problems and damage in addition to causing other forms of discomfort, fatigue, etc.
In some instances, the muffler can be connected to the exhaust within an engine enclosure. In other types of craft, the muffler is better adapted for disposition outside--particularly below--the fuselage. A typical craft that lends itself to the outside or "belly mounting" of the muffler is the "Glasair", manufactured by Stoddar Hamilton Aircraft Inc., Arlington, WA. The muffler provided according to the present invention is well adapted for such installations because its front or nose end is aerodynamically designed. Further it is essentially a cylindrical structure with few if any radial projections, except for the exhaust inlet on some versions. The inventive muffler is seen as a double-walled tubular structure in which the annular space between inner and outer annular walls or casings contains sound-absorbent material. The wall of the inner tubular element or casing has perforations, preferably louvered or finned slots, effecting a pressure wave trap in which the pressure waves are spread radially into the sound-absorbent material. The rear end of the structure is in the form of a cone having its axis coaxial with the annular walls, with its apex rearwardmost. Intermediate its base and apex, the cone is fitted with a circular partition to provide a rear, conical chamber containing sound-absorben material. The wall of the cone intermediate its junction with the outer cylindrical wall of the main body of the muffler is perforated to enable the escape of exhaust. Other versions of the basic concept provide a forward chamber for sound-absorbent material, other types of diffusers and the like, all of which will become apparent as preferred embodiments of the invention are disclosed herein.